1. Field of the Invention
The present invention relates generally to a circuit system for preventing a measuring device from being erroneously operated. More particularly, the present invention relates to a circuit system for preventing a measuring device such as an engine speed measuring device or the like from being erroneously operated due to a bound which is caused in a wave form representative of a sensor detection output when the present engine speed or the like is measured by the measuring device.
2. Description of the Related Art
For example, in a case where the rotational speed of a rotary member Z is measured, sensors (photosensors, magnetic sensors or the like) S constituting a measuring device are arranged in the vicinity of the rotary member Z as well as a rotational shaft X of the rotary member Z in a spaced relationship to count the number of a series of detection signals, as shown in FIG. 3. In addition, the measuring device is provided with a filter circuit 1 including resistors and a capacitor and a Schmitt trigger circuit 2 at its input stage so as to properly shape the wave form of each of the detection signals from the sensors S and moreover prevent an erroneous operation from being performed attributable to a bound component in the detection signal, as shown in FIG. 4.
With the measuring device as constructed in the above-described manner, however, it is necessary that a value of resistance of the respective resistors, a value of capacitance of the capacitor in the filter circuit 1 and characteristics of the Schmitt trigger circuit 2 are individually correctly regulated depending on measuring conditions (kind and characteristics of the sensors, minimum/maximum frequency of a pulse to be measured and other conditions). Further, even though optimum conditions are once settled for the measuring device, there is still a possibility that the bound can not completely be removed and thereby the bound component is unavoidably counted when properties of the sensors used for the measuring device are undesirably deteriorated.